Seems to me there would be more labor in doing the truck than erecting a building. Especially if he is only providing labor and you are still paying for everything. He and a couple of guys could put that 40x50 up in a week or less 3 guys. 50 hours each, = 150 hours. Truck... I would think a "nice" paint job would be 500 hours min. "Restore" who knows depending on what he's expecting you could have a 1000 hours in it.
Seems like its a much better deal for him than you. Especially with your attention to detail and quality of work you do.
Get him to pay you to restore his truck, then pay him with your proceeds to put up a building and put the rest of your cash in the bank.